Cellulosic, synthetic and mixed cellulosic synthetic media in the form of a flexible paper filter have been known for many years. Such media layers have been used as is and have been combined with other filter components to form active filtration elements. Such elements can be made of a variety of macro and microfibers having a range of fiber lengths and diameters. In large part, these layers are made in papermaking machines resulting in a substrate layer typically less than 5 millimeters and most often less than 2 millimeters in thickness. Such thin flexible filter media have found a number of useful applications, however, such layers have limits in their applicability. The ability to achieve certain filtration attributes such as pore size, basis weight, thickness, permeability and efficiency are limited by the manufacturing techniques used to make the paper layers and by the components useful in such layers.
Because aerosols, as an example, may be as small as him diameter or as large as 1 mm (W. Hinds, Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles 8, 2nd ed., © 1999 J. Wiley & Sons), conventional technologies are not suitably flexible to effectively accommodate the range of particle sizes in which aerosols may be encountered in air. Further, aerosols may be present in large concentrations in the air streams in certain filtration applications. For example, in diesel engine blow-by or other heavy duty motor exhaust or industrial filtration applications, it is possible to encounter aerosols in concentrations of 700 ppm to 20,000 ppm. Such concentrations are not filtered with high efficiency using the thin filters of the prior art; thus, multiple layers are usually employed.
Accordingly, a substantial need exists in obtaining an improved filtration layer having a substantial thickness, a defined basis weight, solidity and pore size useful for a variety of filtration processes. One useful technique is to use an aqueous slurry based material such as that disclosed in U.S. Ser. No. 11/267,958. Sugiura et al., U.S. Pat. No. 5,755,963, teach a filter element comprising microfibers made from a slurry to obtain a density gradient structure useful in oil filtration. Nielsen et al., U.S. Pat. No. 5,167,765, teach the use of polyester bicomponent fiber and other fibers in making bonded fibrous wet laid sheets for filtration applications.
Filter media have been configured in a variety of filter units for many different filtration applications. In one application, it has been common to remove liquid aerosol particulate from a mobile gaseous phase such as air by using filter elements comprising cellulosic and cellulosic/synthetic fiber combinations wet-laid flat media that has been corrugated and pleated into a useful cylindrical shaped elements of different heights and diameters. The use of formed media comprised wholly of glass fibers and aqueous or solvent based resins for the purpose of consolidating the fibers mass and providing strength and structural integrity has been practiced. Limitations thereof are specific to fiber size (approximately 0.08 to about 4 microns fiber diameter) and to the fact that a secondary process to apply the resin is necessary to consolidate the filter media and provide structural integrity for the performance and survivability of the element in such demanding applications. Further, these filter media suffer from resin migration during the filter media life, which in turn causes a loss of porosity. In use, this loss of porosity is evidenced by increased pressure drop across the filter, clogging of pores, and cracking of the filter, thus leading to early filter failure. Further, these filter media are brittle and may shatter under challenging conditions, leading to catastrophic failure. Finally, the above filter making technique has a practical limit of about 5.0 mm thickness, such that the ability to make thicker media for use with different aerosols or adaptation for other filtration applications is not provided. Nevertheless, the use of wet-laid paper-like media is prevalent in the application of removing aerosol particulates from airstreams.
Some examples of conventional commercially available filtration media for the separation of aerosols from air are products available from the Porous Media Company of St. Paul, Minn.; Keltec Technolab of Twinsburg, Ohio; ProPure Filtration Company of Tapei, Taiwan; Finite® and Balston® filters made by the Parker Hannifin Corporation of Mayfield, Ohio; Fai Filtri s.r.l. of Pontirolo Nuovo, Italy; Mann+Hummel Group of Ludwigsburg, Germany; and PSI Global Ltd. Of Bowburn Durham, United Kingdom.
A substantial need exists to provide a filter medium having improved efficiency of removal of the liquid aerosol from the air stream and reduced pressure drop, and acceptable basis weight and void volume which leads to an increased useful lifetime in application conditions. A substantial need exists to simplify the construction of such filtration apparatus, thereby decreasing the complication and expense of the manufacturing processes over those currently used. A substantial need also exists to provide a filter media having reduced brittleness that can withstand challenging conditions without shattering. A substantial need also exists to provide a filter medium that does not undergo resin migration during use. A substantial need exists to provide a method of making filter media that is adaptable for other filtration applications, such as the removal of solid particulates from air or the removal of impurities from water. A substantial need also exists for a method of making a filter that is adaptable for entraining materials such as particles and polymers that in turn provide functionality to the filter structure. Finally, there is a substantial need to provide a method of making filter media having multiple layers such that each layer has a different structure or composition, e.g. by varying permeability of the layers or by incorporating particles into a layer. These and other advantages are found in the filtration media disclosed below.